1. Field of the Invention
The present invention relates to a frame synchronization pattern design and synchronization method, by which in a transmitter of a communications system employing an orthogonal frequency division multiplexing (OFDM), a frame synchronization pattern is inserted into the starting part of a symbol frame, and by detecting the frame synchronization pattern in a receiver, synchronization of the OFDM transmitter and receiver is performed.
2. Description of the Related Art
The frame structure of a communications system employing an orthogonal frequency division multiplexing (OFDM) can be broken down into a broadcasting type and a burst type according to the objects of application. In the broadcasting type, since data are continuously received, in order to increase data efficiency a synchronization symbol for symbol frame synchronization is not used. Instead, by using data orthogonal frequency division multiplexing symbols having random patterns, the start point of an OFDM symbol is detected. Meanwhile, in the OFDM burst type that has been used in wireless LAN (local area network) and other applications in recent years, data are not continuously transmitted and are transmitted in units of frames at independent time points. Accordingly, detection of the starting point of a symbol frame should be performed under a situation where frequency offset compensation and channel equalization are not performed. The present invention relates to a symbol pattern design for symbol frame synchronization and a synchronization method using the same that can fit the burst-type transmission method which needs individual frame synchronization for each frame in the OFDM communications system.
The theory on the OFDM, which divides an input bit stream, modulates a plurality of carrier waves corresponding to respective divided parts of the bit stream, and transmits the carrier waves in parallel, has been studied since 1950s but due to complexity in implementation it had not been widely put into practical use. Recently, however, with developments of digital signal processing technologies, such as fast Fourier transform (FFT), and very large scale integration (VLSI) technologies, the major problems of the multiple carrier modulation field in the initial stage, including the huge amount of calculation and demands for use of memories operating at high speed and use of oscillators generating multiple harmonic waves, have been solved. Also, due to the inherent potential advantages of the multiple carrier modulation, including the strong properties against frequency selective fading, capability to make a transmission capacity close to a channel capacity, and ability to guarantee an efficient transmission without a complex equalizer in a channel where inter-symbol interference (ISI) is serious, researches and developments are actively performed and the application range increases gradually.
The OFDM method, a kind of multiple carrier modulation method, had been used only in expensive digital data transmission systems a long while in the initial stage. From the beginning of the 80s, the OFDM method has been adopted as the European digital audio broadcasting (DAB) method and as the European digital video broadcasting for terrestrial (DVB-T) method. Then, International Telecommunications Union-Telecommunication (ITU-T) determined a discrete multi-tone (DMT), which is similar to the OFDM, as standard transmission method for an asymmetric digital subscriber line (ADSL) and a universal ADSL (UADSL). Also, research and developments are actively performed in the wireless local area network (LAN) field such as the US (United States) IEEE 802.11 a (a standard set by the Institute of Electrical and Electronics Engineers) and Europe's broadband radio access network (BRAN), and in a multi-carrier CDMA (MC-CDMA) that connects the OFDM and a code division multiple access (CDMA) method.
The OFDM effectively transmits data by using a plurality of subcarrier waves in the bandwidth of a channel. The subcarrier waves are constructed to maximize the band efficiency, compared to the transmission methods such as frequency division multiplexing (FDM). In the FDM method, the band efficiency is low because subcarrier waves are in different frequency bands and a frequency guard band is arranged so that interference between subcarrier waves can be prevented. However, in the OFDM method, subcarrier waves overlap in a frequency domain with each other such that the band efficiency is maximized, while the orthogonal characteristic is maintained so that other subcarrier waves become null (values are ‘0’) in the central frequency of each subcarrier and interference between subcarrier waves can be prevented.
In the OFDM method, an input signal desired to be transmitted is first converted into multiple low-speed parallel signals in a serial-to-parallel conversion, and then each of the low-speed parallel input signals is modulated into a carrier wave that is in an orthogonal relation with the low-speed parallel signal. The OFDM method is implemented by using inverse fast Fourier transform (IFFT) in a transmission terminal and fast Fourier transform (FFT) in a reception terminal. If the cycle (=1/transmission speed) of each of parallel signals is made to be longer than delay diffusion occurring by multiple path delay when the OFDM is used in fading channel matching for wireless terrestrial broadcasting and the likes, the parallel signal will have a characteristic which is obtained after passing through a frequency non-selective fading channel. Accordingly, by setting an appropriate guard interval, degradation of transmission capability by fading is effectively prevented. In addition, in the OFDM method, data in the form of a bit stream are transmitted in parallel in order to satisfy the demands for transmission of high speed data such as a broadcasting signal, and by doing so the frequency bands are efficiently used and transmission speed increases.
In a earlier art OFDM symbol frame synchronization method, the correlation between a guard interval and the latter half of a symbol is used based on the OFDM symbol of an IFFT output. However, in this method, it is difficult to detect an accurate starting point of a symbol in a multiple path channel environment. That is, inter symbol interference (ISI) occurs and makes the guard interval region different from the latter half of the symbol such that the performance of the method is degraded. Accordingly, in the earlier art method using the guard interval, if the starting point of the symbol in the guard interval is roughly placed, a phase offset occurred by inaccuracy of the symbol starting time is compensated for by an equalizer. However, this rough detection of the starting point causes degradation of performance in using a frequency synchronization algorithm employing the guard interval.
Also, there is a method in which OFDM symbol frame synchronization is performed using a pilot. However, since channel equalization is not performed in this method, the pilot itself cannot be depended on in a channel environment where noise is serious.